In code division multiple access (CDMA) communication systems, multiple communications may be simultaneously sent over a shared frequency spectrum. Each communication is distinguished by the code used to transmit the communication.
In one type of CDMA communication system, the shared spectrum is time divided into frames having a predetermined number of time slots, such as fifteen time slots. This system is referred to as a hybrid CDMA/time division multiple access (TDMA) communication systems. In another type of CDMA system, uplink and downlink communications are restricted to particular time slots. This system is referred to as a time division duplex (TDD) communication system.
In a typical TDD/CDMA communication system, communication data is sent using communication bursts. FIG. 1 illustrates a communication burst 16 having a midamble 20, a guard period 18 and two data fields 22, 24. The data fields 22, 24 carry the data of the communication burst 16. The guard period 18 separates the communication bursts 16 to allow for a difference in arrival times of bursts transmitted from different transmitters. The midamble 20 separates the two data fields 22, 24 and has a known training sequence used to estimate the channel that the communication burst 16 experiences. Using the estimated channel response, data from the data fields 22, 24 is recovered at a receiver. For the third generation partnership (3GPP) wideband CDMA (W-CDMA), based on the burst type, the basic midamble codes used to generate midamble bursts have differing lengths. To illustrate, the basic midamble code for burst type I has 456 chips while burst type II has 192 chips.
FFT is a powerful tool for efficient implementation of a wireless communications receiver. One drawback with FFT implementations is that they are limited to operating on fields with a predetermined length.
It is desirable to have a channel estimator using an FFT engine that has the capabilities to handle channel estimation for various types of bursts.